This invention relates to epoxy resin compositions. In one aspect, the invention relates to methods for imparting flame retardancy to epoxy resin compositions. In a specific aspect, the invention relates to flame-retardant epoxy compositions having suitable uncured viscosity and cured heat resistance for use in reinforced composites.
Present techniques for imparting flame retardance to epoxy resins generally have one or more disadvantages. A common technique is to prepare an epoxy resin, which is conventionally prepared from the reaction of epichlorohydrin with bisphenol-A, from a brominated bisphenol, such as tetrabromobisphenol-A. This can be done by fusing a preformed liquid epoxy resin with a brominated bisphenol in the presence of a fusion catalyst such as a quaternary phosphonium halide. This technique, however, produces a resin which is solid at room temperature and must be diluted with large quantities of solvents in order to reduce its viscosity to a level suitable for impregnation of reinforcing fibers. Furthermore, the necessity for removal of these solvents causes a number of processing difficulties in the manufacture of fiber-reinforced composites. In addition, these resin molecules have long distances between epoxy groups and cure to materials with relatively low crosslink density and low heat resistance.
An alternate route to preparing flame-retardant epoxy resins involves the reaction of tetrabromobisphenol-A directly with epichlorohydrin to produce a low molecular weight brominated epoxy resin. This cured resin exhibits heat resistance similar to that of standard liquid epoxy resins cured with the same curing agents. However, the diglycidyl ether of tetrabromobisphenol-A is a relatively high-melting crystalline solid which tends to crystallize out of mixtures with other epoxy resins and diluents on standing. It is thus difficult to formulate into stable liquid epoxy systems.
Flame-retardant cured epoxy systems can also be prepared by curing standard liquid epoxy resins with halogenated polyphenols such as brominated poly(hydroxystyrene). These solid polymeric curing agents, however, yield resin/curing agent mixtures of such high viscosity that they require substantial quantities of solvents to reduce the viscosity to levels suitable for fiber impregnation at room temperature.
A further route to flame retardancy in epoxy resins is the addition of a nonreactive liquid flame retardant such as tris(2,3-dibromopropyl)phosphate or pentabromodiphenyl ether. This technique, unlike certain ones previously described, generally does not increase the viscosity of liquid epoxy resins and requires no solvent dilution. However, these systems suffer from the disadvantage that the unreactive liquid additive remains in the resin after cure as small molecules which plasticize the cured resin matrix and lower its heat resistance.
Yet another way to prepare a flame-retardant epoxy resin system is to add a high-melting organic powder such as decabromodiphenyl ether or an inorganic powder such as alumina trihydrate. These powders melt at temperatures higher than the glass transition temperature of the epoxy matrix and thus do not generally decrease the heat resistance of the cured systems. However, the solid additives do not dissolve in the liquid resin/curing agent mixture before cure but remain as solid suspensions. During fiber impregnation, the fibers often tend to "filter" the solid particles out of the resin mixture. Agglomeration of the particles may lead to clogging of close-tolerance orifices and create other processing difficulties in obtaining a cured fiber-reinforced composite from such a system.
The ideal flame retardant for epoxy systems would be a low-viscosity liquid which would reduce the viscosity of the resin/curing agent mixture. During resin cure, the flame retardant would either react with the resin in a manner which would not bring about a reduction in the glass transition temperature or heat distortion temperature of the resin, or would polymerize with itself to produce a second phase which would not lower resin properties.
Certain polymerizable flame retardants, such as vinyl bromide and 2,3-dibromopropyl acrylate, are known in the industry. Vinyl bromide has the disadvantage of vaporizing at temperatures customarily used for epoxy curing. Acrylate polymers customarily have glass transition temperatures lower than those of cured epoxy systems. The polymers of brominated acrylates, therefore, would be expected to lower the heat resistance of epoxy resins containing them, since they would be expected to contribute little to network strength when heated above their Tg.
It is therefore an object of the invention to provide a flame-retardant epoxy resin composition. It is a further object to provide a flame-retardant epoxy resin composition which exhibits low uncured viscosity and high cured Tg.